Semiconductor saturable absorbers have found applicability as modelocking elements in solid state lasers for generating extremely short duration optical pulses. These pulses are commonly called ultrashort pulses because they exhibit pulse widths in the picosecond and sub-picosecond ranges. Ultrashort optical pulses are useful for high speed signal processing and data communications.
The saturable absorber allows passive modelocking of a laser when the absorber, which is a nonlinear element, is placed either within the lasing optical cavity or in an external optical cavity coupled to the lasing cavity. Saturable absorbers act as shutters to incident radiation because they can change their opacity as a function of the intensity of the incident radiation at a particular wavelength. A saturable absorber can absorb all weak incident radiation. As the intensity of incident radiation reaches a sufficiently high level, known as the saturation intensity, incident radiation is permitted passage through the saturable absorber. In general, attenuation caused by the absorber is relatively low because the absorber is saturated into a transparent state at the desired wavelength.
Semiconductor saturable absorbers have been fabricated for narrowband and broadband response. Bulk semiconductor material and multiple quantum well heterostructures have been used for narrowband absorption applications while specially graded bandgap multiple quantum well heterostructures have been developed for broadband applications. In the quantum well realizations of such absorber devices, the quantum well heterostructure has been grown on a semiconductor quarter-wave stack reflector. In another embodiment known as an anti-resonant Fabry-Perot saturable absorber, a thin film oxide partial reflector stack was deposited on the quantum well heterostructure to form a Fabry-Perot etalon with the semiconductor quarter-wave stack reflector. For the latter device, the saturable absorber element (MQW) responds to radiation at wavelengths in the anti-resonant portion of the Fabry-Perot etalon response characteristic. This device produces weak coupling with the laser cavity and introduces less loss than other multiple quantum well devices used for modelocking the laser. By the same token, the anti-resonant Fabry-Perot saturable absorber requires significant additional device processing and optimization for its realization.
U.S. Pat. No. 5,627,854, issued to applicant W. H. Knox on May 6, 1997, describes a modelocking solid state laser wherein the modelocking element is a saturable Bragg reflector incorporating one or more quantum wells within a semiconductor quarter wave stack reflector. The resulting nonlinear reflector provides an intensity dependent response which permits it to be used for saturable absorption directly in the main oscillating cavity of a laser.
The saturable modelocking element of the '854 patent has permitted the fabrication of femtosecond modelocked lasers. However for self-starting and stable operation, it has been necessary to incorporate a mechanical tuning element such as a prism and aperture in the cavity of the laser. Such mechanical tuning elements require precise alignment. They increase the size of the device, increase manufacturing cost and present the potential of destroying laser operation by mechanical shock. Accordingly, there is a need for an improved modelocking solid state laser that can achieve self-starting and stable operation without mechanical tuning elements.